How do I rotate an object along it's own axis (windmill)
Actually, as your wheel is not aligned with the global axis anymore, Blender will mess up with the rotation interpolation (Quaternion mode will work better than Euler but it's not perfect either). Also, the animation won’t take your 3D View Pivot Center setting into account, it will use the object's origin.
The easiest way to do it is to keep your wheel aligned with the global axis, parent it to an empty, tilt the empty the way you want, and animate the wheel. The empty parent will somehow be the new world landmark of your object and this time it will work fine.
About the rotation value: you should choose 180°, then, in the Graph Editor, shift E Linear Extrapolation so that it understands that it needs to go indefinitely with a constant speed.